One of the most commonly recognized difficulties associated with airplanes and other aircraft is fueling and defueling. Jet fuel, for example, is a flammable hydrocarbon liquid that can be ignited even in certain ambient conditions, primarily based on temperature and vapor concentration. The temperature at which the vapors of a flammable liquid can ignite is known as the “flash point.” A hazardous vapor concentration is present when a fuel vapor reaches a level known as the lower flammability limit (LFL) or lower explosive limit (LEL). These limits are usually expressed as a percentage by volume. Fuels below the LFL/LEL are considered too lean to burn. If the fuel vapor concentration exceeds the upper flammability limit or upper explosive limit, the fuel is considered too rich to burn. A fuel vapor concentration between these two limits is considered to be in its flammable range and will ignite and burn if exposed to an ignition source.
In addition, hydrocarbons, including jet fuel, may also present toxic or irritant hazards. At high concentrations, jet fuel and other hydrocarbons can affect the nervous system, causing headache, dizziness, and lack of coordination. Exposure to some hydrocarbons may also cause skin irritations if not controlled. Therefore, it is desirable to prevent or minimize any fuel spills when fuelling or defueling an airplane.
Further, defueling a plane is common occurrence. It is often desirable to defuel an aircraft before taking an airplane out of service for maintenance or repair. Therefore, there are a number of airplane defueling systems currently available. However, most defueling systems include gravity draining, resulting in low flow rates and long drain times. In fact, for many large airplanes, depending on the amount of fuel remaining in the tanks, it can take up to seven hours or more to fully drain the fuel from the plane using a gravity dependent system.
Some more recent defueling systems include a vacuum assist to increase the rate of defueling. Vacuum assist defueling systems can evacuate airplane fuel tanks in a fraction of the time normally allocated to a gravity system. However, it is still desirable to reduce or eliminate releases of the aircraft fuel to atmosphere. Accordingly, there is currently a coupler described in U.S. Pat. No. 5,117,876, incorporated in its entirety by this reference, that taps into a main vacuum. The main vacuum provides vacuum pressure which is used to evacuate fuel tanks and seal the coupler to the aircraft drain valves. Thus, the coupler prevents the loss of any fuel to atmosphere, as long as the main vacuum is available. However, if the main vacuum is lost, the seal of the coupler is also compromised, and fuel can potentially leak. Therefore, there is a need for an aircraft-defueling system that minimizes the risk of fuel leaks, even when the main vacuum is not available.